1. Field of the Invention
The present invention relates to a disc player, and more specifically to an apparatus for monitoring a reading position for use in a disc player for playing an information recording disc on which an information signal is recorded together with an identification code for indicating a time axis position.
2. Description of Background Information
Information recording discs such as video discs and digital audio discs (simply referred to "discs" hereinafter) are configured that identification code such as a time code indicating a time axis position (position in a time axis) is recorded in addition to the information signal such as the video signal or an audio signal. Furthermore, in lead-in and lead-out areas respectively in the innermost and outermost track portions of the disc, there are recorded identification codes (lead-in code and lead-out code) for indicating that the present position is in each of these areas.
Disc players for playing this type of recording discs are structured such that an optical light beam is used to trace a recording track of a disc which is constituted by a series of pits formed in the recording surface of the disc, and a recording signal is read-out by detecting the light from the recording surface of the disc. For such operations, it is necessary to use various servo systems, namely, a focus servo system for converging the light beam onto the surface of the disc to form a light spot for reading information (information reading point), a tracking servo system for moving the information reading point so that it follows the recording track accurately, and a spindle servo system for controlling a relative speed of the information reading point with respect to the recording disc.
These servo systems are configured such that a control signal is produced by detecting the light from the information recording surface of the disc, and a control operation is performed by using the produced control signal. Therefore, if the information reading light spot traces a portion where no signal is recorded, erroneous control operations may be pursued so that the servo systems become uncontrollable. In the worst case, mechanisms of the disc player in each part can be destroyed.
Therefore, it is necessary to monitor the position of the information reading point in order that the information reading point will not be moved inwards beyond the lead-in area or outwards beyond the lead-out area. For this purpose, a method has been conventionally utilized in which the arrival of the information reading point at the lead-in area or lead-out area is detected by means of the contents of the identification code (lead-in code or lead-out code) read out from the disc.
However, in the case of this method, the lead-in and lead-out codes may not be read out because of reasons such that the focus servo system is placed in a defocusing condition, or the recorded information is not read-out accurately. In such an event, the information reading point can be driven inwards beyond the lead-in area or outwards beyond the lead-out area.